Porous sintered composite materials

ABSTRACT

The present invention is directed to porous composite materials comprised of a porous base material and a powdered nanoparticle material. The porous base material has the powdered nanoparticle material penetrating a portion of the porous base material; the powdered nanoparticle material within the porous base material may be sintered or interbonded by interfusion to form a porous sintered nanoparticle material within the pores and or on the surfaces of the porous base material. Preferably this porous composite material comprises nanometer sized pores throughout the sintered nanoparticle material. The present invention is also directed to methods of making such composite materials and using them for high surface area catalysts, sensors, in packed bed contaminant removal devices, and as contamination removal membranes for fluids.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from and the benefit of U.S.Provisional Application Serial No. 60/432,910, filed Dec. 12, 2002,titled “Nanoporous Sintered Composite Materials;” U.S. ProvisionalApplication Serial No. 60/455,993, filed Mar. 19, 2003, titled “DepthFiltration of Supercritical Fluids;” and U.S. Provisional ApplicationSerial No. 60/475,729, filed Jun. 4, 2003, titled “Depth Filtration ofSupercritical Fluids and Improvements Thereto,” the contents of whichare each incorporated herein by reference in their entirety.

BACKGROUND

[0002] Porous materials may be obtained by molding and sintering powderscontaining fibrous, dentritic, or spherical shaped precursor particles.The precursor particles are commonly metals, like platinum, or nickel,or their alloys, ceramic materials like alumina, or polymeric materialslike polytetrafluoroethylene. In such porous materials, the strength ofthe material, the size of the pores in the material, and the surfacearea of the material are related to the packing density, the size, theshape, and the composition of the particles making up the powder.Sintering process conditions also affect the strength, pore size, andsurface area of such porous materials. To achieve small pores and highsurface area, the sintering of small diameter particles is preferred.

[0003] In materials with large pore sizes, the size of the pores may befurther reduced using a variety of techniques. For some materials it maybe possible to vapor deposit, electroplate, or electroless plateadditional material into the pores of the base porous material. Thesemethods result in uniform coverage and reduced pore size, but they alsoresults in reduced surface area of the material. Alternatively, a slurryof particles is formed and applied by spraying or brushing the slurryonto the surface of the material and then sintering it after drying.This method does not ensure penetration of the particles into thesubstrate so as to occupy at least a portion of the inner pores. Thismethod results in poor adhesion between the applied slurry and theporous substrate due to differential shrinkage of the slurry powder andthe substrate surface during sintering. Further, this method may notbuild up of a layer or powdered precursor capable of sintering to form aporous structure.

[0004] Porous and high surface area materials are used in, catalysis,gas sensing, and filtration. For example, finely divided noble metals oralloys (Pd, Pt, and Rh) deposited on a porous ceramic or metal substratemay be used as a combustion catalyst to thermally decompose hydrocarbonsgases; these types of catalysts may also be used to remove NO_(x) and COfrom exhaust gases. Porous materials may be used as electrodes in fuelcells where the electrolyte in the cell is a solid polymer. For properoperation, the polymer electrolyte in these fuel cells needs to bemaintained in a hydrated form to prevent loss of ionic conductionthrough the electrolyte. In order to maintain membrane hydration andsuitable reactions at the cell electrodes, one or more of the cell'selectrodes may be made from very small metal particles (usually 2-5 nmdiameter) that are distributed on, and supported by, larger conductingparticles. These supported metal particles are formed into high surfacearea electrodes that are porous in order to optimize contact between thereactant gas, the electrolyte, and the metal catalyst. Pellistors aregas sensors having a porous metal electrode on a ceramic, (i.e. a ThO₂and Al₂O₃ ceramic pellet coated with a porous catalytic metal like Pd orPt), that reacts with flammable gases to generate heat which is detectedby an RTD embedded in the ceramic pellet. The detection limit for thesesensors is related to the amount of heat generated by the decompositionreaction; this depends on the active area of the porous metal electrode.

[0005] Sintered ceramic and metal gas filters typically have pore sizesin the 1-10 um range and can remove particles down to 0.003 micron witha log retention value of greater than 9. In gases, particle capture isby diffision and interception with the filter surface. Because of thelow viscosity of gases, the filters are able to flow large volumes ofgas with nominal pressure drop across the membrane. In liquids thesesame filters would only remove particles in the 1-10 um range with anLRV of about 2 because sieving is the dominant mechanism for particleremoval or capture in liquids. Because of the higher viscosity of theliquid, the pressure drop across the same filters for a given volumetricflow rate would be greater for a liquid than for a gas. Supercriticalfluids, those materials whose temperature and pressure are above thecritical values, have properties that are intermediate between those ofgases and liquids. Supercritical fluids generally retain the solvationproperties and densities of the liquid while having gas like viscositiesand surface tensions. Because of a supercritical fluid's solvatingproperties, it interacts with both the particle and filter surfaces,particles in supercritical fluids are preferably removed from the fluidby sieving rather than diffusion and interception. Because of their gaslike viscosity and surface tension, supercritical fluids will have apressure drops across the same filter more like a gas than a liquid. Itis possible that smaller pores, nanometer sized pores or smaller, may bedesigned into the filter to capture nanometer and sub-nanometer sizeparticles by sieving but without greatly increasing the pressure drop ofthe filter.

[0006] It would be desirable to have a mechanically strong, high surfacearea, material with small pores. Further, it would be desirable to beable to make objects with these properties with different materials andin a variety of shapes and sizes.

SUMMARY

[0007] One embodiment of the present invention is a porous compositematerial, comprised of a porous base material and a powderednanoparticle material. The porous base material has the powderednanoparticle material penetrating a portion of the porous base material.The powdered nanoparticles penetrating the porous base material may thenbe interbonded to one another by interfusion or sintered to form aporous sintered nanoparticle material within the pores of the porousbase material. Preferably this sintered porous composite materialincludes nanometer and sub-nanometer sized pores throughout thethickness of sintered nanoparticle composite material, permitting flowof a fluid therethrough, and preferably removing particles from thefluid by sieving. The pores of the porous sintered nanoparticle materialare smaller than the base material, permit a flow of fluid through theporous sintered nanoparticle material, and may have their largestdimension less than about 5000 nanometers; preferably less than 1000nanometers, more preferably less than 200 nanometers, and even morepreferably less than 50 nanometers. The porous sintered compositematerial may be bonded to a housing for connection to a fluid flowcircuit, the bond between the housing and the porous sintered compositematerial providing a substantially uniform particle retention across thesintered porous composite material joined to the housing.

[0008] Alternatively, the powdered nanoparticle material may be allowedto penetrate a portion of the porous base and then accumulate on one ormore surfaces of the porous base to form a layer of the nanoparticlematerial. After sintering, a sintered porous composite material thatincludes a porous sintered nanoparticle material within the pores of thebase and a porous layer of sintered nanoparticle material on one or moresurfaces of the porous base is formed. The porous layer of sinterednanoparticle material forms a continuous structure with the poroussintered nanoparticle material within the pores of the base. Thethickness of the porous composite material includes the porous basematerial and one or more sintered porous nanoparticle material layers.The powdered material in the pores of the base may sinter to the basematerial or only sinter to itself. The powdered nanoparticles in thefine layer on the one or more surfaces of the porous base may sinter tothe base material, sintered to both, or sintered only to thenanoparticle material. Preferably the sintered porous composite materialcomprises nanometer and sub-nanometer sized pores throughout thesintered porous nanoparticle material. Preferably this sintered porouscomposite material comprises nanometer and sub-nanometer sized poresthroughout the sintered nanoparticle composite material, permits a flowof a fluid therethrough, and preferably removes particles orcontaminants from the fluid by sieving. The sintered porous compositematerial may also include a supercritical fluid within the pores of thematerial. The pores of the porous sintered nanoparticle material aresmaller than the base material, permit a flow of fluid through theporous sintered nanoparticle material, and may have their largestdimension less than about 5000 nanometers; preferably less than 1000nanometers, more preferably less than 200 nanometers, and even morepreferably less than 50 nanometers. The porous composite material maycomprise layers of different nanoparticle materials including but notlimited to varying size, shape, and composition. The porous sinteredcomposite material may be bonded to a housing. Preferably the bondbetween the housing and the porous sintered composite material retainsthe integrity of the porous sintered composite material and provides asubstantially uniform particle retention across the sintered porouscomposite material joined to the housing.

[0009] The powdered nanoparticle materials making up the composite mayhave diameters less than about 1000 nanometers. Like the porous basematerials, these nanoparticle materials may be metals, metal alloys,ceramics, thermoplastics, or mixtures of these materials. The startingnanoparticles should be able to penetrate into the porous base material,and may have shapes including but not limited to spheres, dendrites,fibers, or mixtures of these particles. Preferred powdered nanoparticlematerials include dendrites of nickel or alloys containing nickel.

[0010] A sintered porous composite material may be made into anelectrode element, a catalyst element, or a filter element. The elementmay be bonded to a housing or other suitable structure that maintainsthe integrity of the sintered porous composite material, providesmechanical support, and permit connection of the element into a fluidsystem.

[0011] In one embodiment of the present invention a sintered porouscomposite material or other filter element is welded or otherwisesecured into a housing that is then filled with a powder composition.The porous sintered composite material may be bonded to the housing suchthat the bond between the housing and the porous sintered compositematerial retains the integrity of the porous sintered composite materialand provides a substantially uniform particle retention across thesintered porous composite material joined to the housing. The powder maybe distributed over the filter element within the housing using suitabletechniques until a packing density and the mass of powder are sufficientto remove particles or other contaminants from the fluid with which itwill be used. Various configurations of the bed, including but notlimited to graded particle size beds, baffles, as well as differentparticle materials comprising the bed are possible.

[0012] Another embodiment of the present invention is a method of makinga porous composite material. The composite material is made by flowing asource of nanoparticle material suspended in a fluid medium into orthrough a porous base substrate and capturing a portion of thenanoparticle material particles within the porous base object. Thecaptured nanoparticle material and porous base object may be sintered orinterbonded by interfusion to form a sintered porous composite material.Depending upon its intended use, it may be desirable to allow thenanoparticle material to penetrate and accumulate as a porous layer onone or more surfaces of the porous base object. When the accumulatedlayer of nanoparticle material has reached its desired weight orthickness, the flow of nanoparticle material particles is stopped. Theporous base with the accumulated layer of nanoparticle material are thensintered to form a sintered porous composite material including a layer,and preferably a fine layer, of nanoparticle material atop the poroussubstrate which penetrates a portion of the base material and forms acontinuous structure with a porous sintered nanoparticle material withinthe pores of the base. Preferably this sintered porous compositematerial comprises nanometer and sub-nanometer sized pores throughoutthe sintered nanoparticle composite material and permits a flow of afluid therethrough, and preferably retains particles and removes themfrom the fluid by sieving. The sintered porous composite material mayalso include a supercritical fluid within the pores of the material.

[0013] Another embodiment of the present invention is a method of makinga porous composite material and a filter bed. The porous compositematerial may be made by flowing a source of nanoparticle materialsuspended in a fluid medium into or through a porous base substrate andcapturing a portion of the nanoparticle material particles within theporous base object. The captured nanoparticle material and porous baseobject may be sintered or interbonded by interfusion to form thesintered porous composite material. Depending upon its intended use, itmay be desirable to allow the nanoparticle material to penetrate andaccumulate on one or more surfaces of the porous base object. When theaccumulated layer of nanoparticle material has reached its desiredweight or thickness, the flow of nanoparticle material particles isstopped and the element may be sintered. The sintered porous compositematerial element may be bonded or welded into a housing such that theporosity and integrity of the porous sintered composite material isretained. Micrometer or nanometer sized material may then be placedaround the filter element to form a pack bed. Various configurations ofthe bed, including but not limited to graded particle size beds,baffles, as well as different particle materials can be made.

[0014] In another embodiment, the sintered porous composite material ofthis invention is characterized in that it has an LRV of at least about2, and preferably 4 for about a 0.2 μm diameter PSL bead particlechallenge by sieving in water. It may be characterized by having apressure coefficient in nitrogen gas of less than about 250 (psi cm2)/slpm, more preferably less than about 125 (psi cm²)/slpm, and evenmore preferably less than about 30 (psi cm²)/slpm. The material is ableto withstand a differential pressure across the sintered porouscomposite material membrane of greater than 60 psi, and more preferablygreater than about 400 psi. Even more preferably, the sintered porouscomposite material of the present invention is characterized in that ithas an LRV of at least about 2, and preferably 4, for about a 0.05 μmdiameter PSL bead particle challenge by sieving in water. The materialmay have a pressure coefficient in nitrogen gas of less than about 250(psi cm²)/slpm, more preferably less than about 125 (psi cm²)/slpm, andeven more preferably less than about 30 (psi cm²)/slpm. The material isable to withstand a differential pressure across the porous compositematerial membrane, the porous base providing support for the poroussintered nanoparticle material, of greater than 60 psi, and morepreferably greater than about 800 psi.

[0015] The depth of penetration of the powdered nanoparticle materialinto the porous base object to form the a porous composite material maybe controlled by the velocity of the fluid medium flowing through theporous base object as well as the particle capture efficiency of theporous base. The amount of powdered nanoparticle material accumulatedwithin the porous base or on the surface of the porous base may becontrolled by the concentration of the particles in the slurry, thetotal volumetric flow through the porous base, the state of the fluiditself (i.e.; gas, liquid, or supercritical fluid) and the size of theparticles. The nanoparticles may penetrate the porous base in a rangefrom below the top surface through the entire depth of the porous baseobject.

[0016] In one method the porous composite material is made by flowing asource of un-agglomerated powdered nanoparticle material into or throughthe porous base material. At least a portion of these particles arecaptured within the pores, or within the pores and on top of one or moresurfaces of the base material. The captured powdered nanoparticlematerial and base material are sintered to form the sintered porouscomposite material. The powdered nanoparticle material suspended in thefluid may be delivered into or flowed through the porous base materialby atomizing or making a slurry of the particles in a fluid.Alternatively, a source of the powdered nanoparticle material may beisostatically pressed into the porous base material.

[0017] The formed sintered porous composite material may be used forfiltering a fluid to remove suspended particles or contaminants from thefluid. The sintered porous composite material may also include asupercritical fluid within the pores of the material. A method forfiltering the fluid includes providing a sintered porous compositeelement including a porous base and a sintered porous nanoparticlematerial penetrating the base pores and forming a porous layer on one ormore surfaces of the base, and flowing a fluid with contaminants, likeparticles, through the element to remove one or more particles from thefluid. Preferably the particles are removed by sieving filtration. Thesintered porous composite element may provide sieving filtration forsmall particles and would be advantageous in the case of filtration ofsupercritical fluids. The porous base of the element to providesmechanical support and allows the porous layer of sintered nanoparticlematerial on one or more surfaces of the base to withstand the highpressures in the supercritical fluid system. The sintered porousnanoparticle material within the pores of the base and atop the surfacesof the base may provide sieving filtration to various fluids; the lowviscosity and surface tension of a supercritical fluid may minimizepressure drop across such a filter element. The high surface area of thesintered porous composite material may provide high particle retentionand capacity, reduced pressure drop, and enable small footprintcomponents to be made. A small diameter component is mechanicallyadvantageous for any pressurized fluid system. This is because as theoverall pressure of the system increases, the wall thickness ofcomponent must also increase to withstand such pressures; this increasesmaterial costs and also the size of the components.

[0018] The sintered porous composite material element could be used toremove materials like particles or molecular contaminants from gases byretention, chemically bonding, or catalytic action of the sinteredporous nanoparticle material with the fluid. Interaction of the sinteredporous composite material element may be by chemisorption orphysisorption of the contaminants in the fluid with these high surfacearea materials. A method for removing material from a fluid includesflowing a fluid having the material or molecular contaminants in thefluid through a sintered porous composite material element wherein thesintered porous composite material element removes the material from thefluid. The material can be removed by particle capture, chemisorption,physisorption, or a combination of these. The sintered porous compositematerial may also include a supercritical fluid within the pores of theporous sintered composite material and may be used to removecontaminants from the fluid.

[0019] Another embodiment of the present invention is a supercriticalfluid with less than 50 particles per milliliter, and preferably lessthan 5 particles per milliliter, the particles having a size of 0.2micrometers or less and preferably 0.05 micrometers or less. Preferablythe number of particles greater than about 0.2 micron in size remainingon a substrate cleaned with a about 5 liters of supercritical carbondioxide fluid filtered with a sintered porous composite material or adevice including a sintered porous composite material element of thepresent invention and a packed bed of material is less than about 300counts on a 200 mm diameter substrate, and more preferably less about100 counts on a 200 mm diameter substrate.

[0020] Because of the small pore size and high surface area of theporous composite materials and sintered porous composite materials ofthe present invention, they may provide sieving filtration forsupercritical fluids with low pressure drop and high particle loadingcapacity. Prior to sintering these porous composite materials have asurface area in the range of 2-5 ml²/gram or more; following sinteringthe surface area is about 1 m²/gram or more. The porous compositemateials may also provide for improved detection limits for gas sensors,for example pellistors, that can use the sintered composite porousmaterial as an electrode. The high surface area of the sintered porousnanoparticle material provides numerous sites for catalyticdecomposition of the target gas which creates more heat for the thermalsensor to detect.

[0021] Advantageously, embodiments of the present invention do notrequire the use of a binder to form the sintered porous compositematerials so that high purity membranes, catalysts, and sensor elementsmay be formed with out the need for burning off the residues of thesebinders. In addition, a porous sintered nanoparticle material within thepores of the base material forming a continuous structure with a poroussintered nanoparticles layer on one or more surfaces of the base andhaving interconnected nanometer and sub-nanometer sized pores, may beformed as a single layer in a single deposition step with the porousbase material.

BRIEF DESCRIPTION OF THE FIGURES

[0022] In part, other aspects, features, benefits and advantages of theembodiments of the present invention will be apparent with regard to thefollowing description, appended claims and accompanying drawings where:

[0023]FIG. 1 is an illustration of a cross section of powderednanoparticle material deposited on the green form of a porous basematerial;

[0024]FIG. 2 is an illustration of a cross section of a powderednanoparticle material following isostatic pressing of the powderednanoparticle material into a porous base material to form a porouscomposite material;

[0025]FIG. 3 is a illustration of the cross section of a sintered porouscomposite material of the present invention;

[0026]FIG. 4 is a plot comparing the Flow delta P and the bubble pointfor a sintered composite porous material made by isostatically pressinga nickel nanoparticle powder into a green form of Example 2 andsintering;

[0027]FIG. 5 is a plot illustrating the particle retention of PSL beadsin water by the sintered porous composite material made by isostaticallypressing a nickel nanoparticle powder into a green form of Example 2 andsintering;

[0028]FIG. 6 is a schematic illustration of a packed bed device of thepresent invention comprising a coarse inlet filter element, a packed bedof materials to provide depth filtration and or purification, and anoutlet sintered porous composite filter element of the presentinvention;

[0029]FIG. 7 is data illustrating the reduction in particle counts onsubstrates using the porous sintered composite filter element and packbed embodiments of the present invention to remove contaminants from asupercritical fluid. The particle counts are for particles >0.2 micronon a substrate determined using a KLA-Tencor surfscan. The results arefor 200 mm substrates and cleaning is performed using about 5 liters ofsupercritical CO₂ per run;

[0030]FIG. 8 is a graph illustrating the pore symmetry test in water ofa porous sintered composite material filter element of the presentinvention welded into a housing where the edge of the weld area wassealed;

[0031]FIG. 9 is a graph illustrating the particle retention of PSL beadsin water using a sintered porous composite material filter element ofthe present invention welded into a housing where the edge of the weldarea was sealed;

[0032]FIG. 10 is a plot of the pressure drop versus flow rate usingwater as a fluid for a sintered porous composite material filter elementof the present invention welded into a housing where the edge of theweld area was sealed;

[0033]FIG. 11 is a plot of the pressure drop versus flow rate usingsupercritical CO₂ for a porous sintered composite material filterelement of the present invention welded into a housing where the edge ofthe weld area was sealed;

[0034]FIG. 12 is a graph illustrating the pore symmetry test of a poroussintered composite material filter element of Example 9;

[0035]FIG. 13 is a graph illustrating the particle retention of PSLbeads in water using a sintered porous composite material filter elementof Example 9;

[0036]FIG. 14 is a graph illustrating the pressure drop versus mass flowrate of supercritical CO₂ for the porous sintered composite materialfilter element and the packed bed of material of Example 6;

DETAILED DESCRIPTION

[0037] Before the present compositions and methods are described, it isto be understood that this invention is not limited to the particularmolecules, compositions, methodologies or protocols described, as thesemay vary. It is also to be understood that the terminology used in thedescription is for the purpose of describing the particular versions orembodiments only, and is not intended to limit the scope of the presentinvention which will be limited only by the appended claims.

[0038] It must also be noted that as used herein and in the appendedclaims, the singular forms “a”, “an”, and “the” include plural referenceunless the context clearly dictates otherwise. Thus, for example,reference to a “particle” is a reference to one or more particle andequivalents thereof known to those skilled in the art, and so forth.Unless defined otherwise, all technical and scientific terms used hereinhave the same meanings as commonly understood by one of ordinary skillin the art. Although any methods and materials similar or equivalent tothose described herein can be used in the practice or testing ofembodiments of the present invention, the preferred methods, devices,and materials are now described. All publications mentioned herein areincorporated by reference. Nothing herein is to be construed as anadmission that the invention is not entitled to antedate such disclosureby virtue of prior invention.

[0039] Embodiments of the present invention are sintered porouscomposite materials that include a porous base material and a layer ofporous sintered nanoparticle material on one or more surfaces of theporous base and penetrating a portion of the porous base material. Theporous sintered nanoparticle material has pores smaller than the poresin the porous base material. The porous sintered nanoparticle materiallayer on top of the base forms a continuous structure with the poroussintered nanoparticle material within the pores of the base. Preferablythe sintered porous composite material includes nanometer andsub-nanometer sized pores throughout the sintered porous nanoparticlematerial that allow a fluid to flow through the porous sinteredcomposite material and particles to be retained by the sintered porouscomposite material. The porous sintered composite material may include asupercritical fluid within its pores. The porous sintered compositematerial may be bonded to a housing for connection to a fluid flowcircuit, the bond between the housing and the porous sintered compositematerial providing a substantially uniform particle retention across thesintered porous composite material joined to the housing.

[0040] The porous base material used for preparing the porous compositematerials may be a metal, a ceramic, a polymeric material or a compositeof these. The porous base may be sintered or a green form of suitablypressed powders. The pores of the base material are interconnected topermit fluid flow and the structure of the porous base may be symmetric,asymmetric, or other geometries known to those skilled in the art.Examples of such porous base materials include but are not limited tometal filter substrates which have an LRV of 9 or more in gases for0.003 μm particles available from Mykrolis Corporation, Billerica,Mass., and Ni-based metal porous body commercially available as “CELMET”® prepared by Sumitomo Electric Ind., Ltd;. polymers such as sinteredTeflon® and polyethylene from Porex, Fairburn, Ga.; flat sheet polymericmembranes are available from WL Gore, Elkton, Md.; porous ceramicelements are available from Filterite, Timonium, Md. The pore size ordiameter for the porous base may be but is not limited to the range ofabout 0.05 to 100 microns, preferably from 0.05 to 50 microns, and morepreferably 0.5 to 10 microns. The porous base may have a thickness suchthat it can mechanically support porous sintered composite material atthe pressures and temperatures expected during use. Preferably themembranes have a thickness of from about 0.03 inches to about 0.1inches. These membranes may also be characterized by the size particleused to make the porous base as well as their porosity. Particles usedto make such porous base materials may range from 0.05 to 100 microns,preferably 0.5 to 10 microns. The porosity of these base materials mayrange from about 45 to about 70%. It is desirable that the pore size ofthe porous base material be such that the nanoparticles penetrate aportion of the porous base material. The density of the porous base maybe less than the bulk density of the material that makes up the base.

[0041] The nanoparticles used to form the composite material may be, butare not limited to spherical, dentritic (as described in U.S. Pat. No.5,814,272 incorporated by reference in its entirety), fibrous, orcombinations of these shapes. Other regular or irregularly shapedparticles may also be used to make the porous composite materials andsintered porous composite materials. The composition of thenanoparticles may be metals or metal alloys. Examples of useful metalsand alloys include but are not limited to copper, nickel, nickel alloys,molybdenum, stainless steels, chromium, chromium alloys, and Hastalloy®.Ceramic or metal oxide powders useful for making the porous compositematerials include but are not limited to alumina, silica, zeolites,titanium dioxide, and cerium dioxide. PFTE materials such as Teflon® 307A, with for example a 0.16 micron nominal diameter, may also be used andare available from Dupont in the form of aqueous dispersions.Thermoplastics such as ultra high molecular weight polyethylene,poly(tetrafluoroethylene-co-perfluro(alkylvinylether)),(poly(PTFE-co-PFVAE)), orpoly(tetrafluoroethylene-co-hexafluoropropylene) or blends of thesematerials, available from Dupont, may also be used. Ceramic and metallicnanoparticle powders are available from Nanostructured & AmorphousMaterials, Inc., Los Alomos, N. Mex. Nanoparticles may also be made byother methods including laser ablation of microspheres as described inU.S. Pat. No. 5,585,020 and incorporated herein by reference in itsentirety.

[0042] Particle sizes for the powdered nanoparticle material used toform the sintered composite porous material are chosen withconsideration for the pore size of the substrate and the desired poresize for the final sintered porous composite material. Generally, thesmaller the desired pore size and the higher the surface area, thesmaller the nanoparticle material that should be used to make thesintered porous composite material. The particle size distribution maybe less than 25% rms of the particles in the sample. In someembodiments, the particle distribution may be less than 5 percent.Particles may have a diameter less than about 1000 nm, preferably lessthan 500 nm, more preferably less than 100 nm, and even more preferablyless than 50 nm. Where non-spherical particles are used, the size may betaken as those corresponding to the largest dimension of the particle.Porous particles may also be used, for example mossy zinc or spongynickel.

[0043] The porous composite material may be formed by suspending thepowdered nanoparticle material into a fluid to form a slurry. Suitablefluids include but are not limited to air, nitrogen gas, water, ethanolwater mixtures, and supercritical fluids. Preferably the slurry iscomprised of non-agglomerated particles. The suspension ofnon-agglomerated particles may be formed by coating the particles with asuitable capping layer or adding a surfactant to the fluid.Alternatively, stirring the slurry and allowing the larger particles tosettle may give a relatively uniform suspension of substantiallynon-agglomerated particles. The slurry, and preferably thenon-agglomerated suspension, of nanoparticle materials is made to flowinto or through the porous support or base material where at least aportion of the particles is retained in the porous base membrane.Alternatively, the slurry is made to flow into or through the membranewhere a portion is retained by the porous base, and is also accumulatedas a layer on top of one or more surface of the porous base. Flow of thesuspended nanoparticle materials is stopped when the mass of materialretained by the porous base and accumulated on its one or more surfacesis sufficient to provide a sintered porous composite with propertiessuitable for its intended use. This may include but are not limited to ahigh surface area, desired pore size, particle retention, catalytic orchemisoption activity, pressure drop or a combination of these. Thepenetration depth of the nanoparticles into the porous base may bechanged by controlling the fluid velocity and size of particlesdelivered to the porous base and the state of the fluid itself. Theliquid suspending the nanoparticles is preferably removed from theporous base and nanoparticles prior to sintering. It is preferred thatthe solvent from the deposited nanoparticle material be removed slowlyto prevent cracking of the accumulated nanoparticle layer on the porousbase.

[0044] The powdered nanoparticle materials may also be made to flow intoor through the porous base by transporting the particles in a gas phase.Nanoparticles may be formed by gas phase nucleation of reactive gases,by flame reactors, or by spray pyrolysis. Nanoparticles formed by laserablation of a surface material or laser ablated micro-particles may alsobe transported by a carrier gas into the porous base. Alternatively, aliquid suspension of nanoparticles is made into an aerosol using anultrasonic atomizer (Sonics, Newton, Conn.) or a gas nebulizer(Meinhard, Santa Ana, Calif.). The liquid is evaporated from theparticles after aerosol formation to yield solvent free particles whichcan be made to flow by a carrier gas through the porous base. Thepenetration depth of the nanoparticles into the porous base andaccumulation of nanoparticles on its surfaces may be changed bycontrolling the velocity of the carrier gas through the porous base. Theamount of material deposited on, or accumulated on, the porous basesubstrate may be determined by mass change of the porous base.

[0045] One or more surfaces of the porous substrate may havenanoparticles accumulated on them to form a layer, and preferably a finelayer, of the nanoparticle penetrating a portion of and atop of thesurfaces of the porous substrate. Mixtures of nanoparticles with variousshapes, sizes, and composition may be made to flow through the porousbase or may be isostatically pressed into a green form, a porous basemembrane, or a sintered frit. The layer of nanoparticle material atopthe porous substrate base may be sintered to form a sintered porousnanoparticle material layer. The sintered porous nanoparticle materiallayer penetrates a portion of the base and forms a continuous porousstructure with the sintered nanoparticle material within the pores ofthe base material. Alternating layers or nanoparticles with differingshapes, sizes and composition may be built up by sequencing thedeposition steps, or by repeating deposition steps after sintering. Theamount of nanoparticle powder in the pores of the base, or the thicknessof the fine nanoparticle layer on the surface of the base, may be variedby the nanoparticle deposition to control the pressure drop andretention characteristics of the final sintered porous compositematerial. Preferably the sintered porous nanoparticle layer on top ofthe porous substrate has a thickness less than 1000 microns, preferablyless than 500 microns, and more preferably less than 100 microns andeven more preferably less than about 10 microns. Multiple layers ofnanoparticle material may be formed with the porous base, and each layermay have a different material composition or nanoparticle size. Thethickness of the porous composite material includes the porous basematerial and the sintered porous nanoparticle material layer. Thethickness of the porous composite material may be varied to alterpressure drop or retention by changing the thickness of the porous baseand or the sintered porous nanoparticle material layer.

[0046] It is preferably that the pores of the sintered porous compositematerial be smaller than the porous base material. Preferably thissintered porous composite material comprises nanometer or sub-nanometersized pores throughout the sintered porous nanoparticle material layerand the sintered nanoparticle material within the pores of the basematerial. The pores may be characterized in that they provide particleretention by sieving and have an LRV greater than 2 for of 0.2 micron orlarger particles, preferably an LRV or greater than 4 for 0.2 micron orlarger particle; more preferably an LRV of greater than 2 for 0.05micron or larger particles, and most preferably an LRV of greater than 4for 0.05 micron or larger particles. The pores of the sintered porousnanoparticle material, which permit a flow of fluid through the pores,may also be characterized their smallest aspect is less than about 1000nanometers, preferably less than about 200 nanometers, and morepreferably less than about 50 nanometers.

[0047] The density of the sintered porous nanoparticle material layer,for example as illustrated by the material from 340 to 350 in FIG. 3, ontop of the one or more surfaces of the porous base preferably provides asufficient contact surface area for catalysis, gas sensing, fluidfiltration, or a combination of these and minimizes the pressure dropacross the sintered porous composite material. This sintered porousnanoparticle layer on top of the porous substrate has a thickness lessthan 1000 microns, preferably less than 500 microns, and more preferablyless than 100 microns and even more preferably less than about 10microns. The density of the sintered porous composite material may becharacterized in that has an LRV of at least about 2, for about a 0.2 μmdiameter PSL bead particle challenge in water by sieving, preferably anLRV of at least about 4 for about a 0.2 μm diameter PSL bead particlechallenge in water by sieving, more preferably an LRV of at least 2 fora 0.05 μm diameter PSL bead particle challenge in water by sieving andeven more preferably an LRV of at least 4 for a 0.05 μm diameter PSLbead particle challenge in water by sieving. The porous sinteredcomposite material may be characterized by having a pressure coefficientin nitrogen gas of less than about 250 (psi cm²)/slpm, more preferablyless than about 125 (psi cm²)/slpm, and even more preferably less thanabout 30 (psi cm²)/slpm. The sintered porous composite material isfurther characterized in that it is able to withstand a differentialpressure across the membrane, the porous base providing support for theporous sintered nanoparticle material, of greater than 60 psi, and morepreferably greater than about 400 psi. The density of the sinterednanoporous material layer, or fine layer, on the surface of the porousbase can be in the range of from 3 to 6 g/cm³; for the porous sinteredcomposite material with an LRV of at least 2 for 0.2 micron particlesthe density of the sintered nanoporous material layer is 3 to 4.5 g/cm³,and is preferably from about 3.8 to 4.2 g/cm³; for the porous sinteredcomposite material with an LRV of at least 2 for 0.05 micron particlesthe density of the sintered nanoporous material layer is 4.5 to 6 g/cm³,and is preferably from about 5 to 5.5 g/cm³

[0048] Without wishing to be bound by theory, the sintered porouscomposite materials of the present invention may be characterized by thepressure loss across the membrane which may be related by aproportionality constant K (the pressure coefficient) to the area of themembrane, the thickness of the membrane, the size, shape, anddistribution of pores in the membrane, and the flow of fluid through themembrane. Using the relationship (1): $\begin{matrix}{{\Delta \quad p} = {K\left( \frac{Q}{A} \right)}} & (1)\end{matrix}$

[0049] where K is the pressure coefficient, Q is a nitrogen gas flow(slpm), and A is the area of the membrane (cm²), and Δp is the pressuredrop (psi); the porous composite material of Example 2 has a K value ofabout 13.5 (psi·cm²)/slpm and the porous composite material of Example 9has a K value of about 21.5 (psi·cm²)/slpm. One skilled in the art wouldknow that the porous composite material membrane properties such as butnot limited to the area of the membrane, the thickness of the membrane,the size, shape, and distribution of pores in the membrane, and theviscosity of fluid through the membrane may be changed to affect theproportionality constant K. For example, the thickness of the sinteredporous nanoparticle material layer (which is approximately linearlyrelated to pressure drop) may be increased or the porosity of the poroussintered nanoparticle material layer decreased to provide a membranewith greater resistance to the flow of fluid, and as a result, a higherpressure coefficient. Although the porous composite materials of thepresent invention are not limited by any value of the pressurecoefficient, porous composite materials of the present invention maypreferably have pressure coefficients in nitrogen gas of less than about250 (psi cm²)/slpm, more preferably less than about 125 (psi cm²)/slpm,and even more preferably less than about 30 (psi cm²)/slpm.

[0050] The sintered porous composite material includes a porous basematerial and a layer of porous sintered nanoparticle material on top ofon or more surfaces of the porous base material and penetrating aportion of the base material to form a continuous structure with asintered porous nanoparticle material within the pores of the base. Thelayer of porous sintered nanoparticle material may be on one or moresurfaces of the porous base and penetrates a portion of the porous basematerial to form a porous and preferably a nanoporous material with thepores of the base material. It may be used for flowing a fluid throughfor catalysis, as part of a sensor, removing particles or contaminantsfrom the fluid or a combination of these. The porous sinterednanoparticle material may be characterized in that its pores are smallerthan the pores in the porous base material. The sintered porouscomposite material may be further characterized in that is has an LRV ofat least 2 for a 0.2 μm PSL bead particle challenge in water by sieving,a pressure coefficient in nitrogen gas of less than about 250 (psicm²)/slpm, more preferably less than about 125 (psi cm²)/slpm, and evenmore preferably less than about 30 (psi cm²)/slpm. The sintered porouscomposite material can support a differential pressure across thematerial of greater than 60 psi. The sintered porous composite materialhas an LRV of at least 2 for a 0.2 μm particle challenge in water,preferably it has an LRV of at least 2 for a 0.05 μm particle challengein water by sieving; and even more preferably it has an LRV of at least4 for a 0.05 μm particle challenge in water by sieving.

[0051] Porous composite materials of the present invention may also bemade by isostatically pressing nanoparticles into a porous base frit orinto an unsintered green form of a porous base material as illustratedin FIG. 1. The green form of the porous base material is made in a firststep using methods well known in the art. The green form is then placedinto a second container with dried nanoparticles which are theisostatically pressed into the green form. The mandrel supporting thegreen form may be porous and enable a flow of nanoparticles in a gasinto the green form. As shown by the non-limiting illustration in FIG.1, porous composite materials may be formed by isostatically pressing180 powdered nanoparticles 130 into microporous and larger pore size 100base materials 120 having a top surface 150 and a bottom surface 170.Preferably upon isostatically pressing 180 non-spherical shaped powders130, the powders penetrate to a depth 160 and pack into the pores 100 ofthe base material and interlock with each other. The powderednanoparticles may then be sintered or interbonded by interfusion to forma sintered porous nanoparticle material within the pores of the base.When used as a base material, a frit contributes less pressure drop to asintered porous composite material filter element than microporous basematerials due to the frit's larger pore size.

[0052] While the porous composite materials described herein may be usedfor particle removal and filtration, preferably they are sintered tobond the nanoparticle materials to improve their mechanical strength andprevent nanoparticles from being dislodged from the porous base. Theporous base material containing entrained nanoparticles, or the porousbase material containing entrained nanoparticles and nanoparticles onthe one or more surfaces of the porous base material may be sintered inan oven to form sintered porous composite materials. For purposes ofthis description, the powdered nanoparticles material may be interbondedto one another by interfusion or equivalently sintered to one another,to form a porous sintered nanoparticle material as a layer atop theporous base or within the pores of the base. The green form withnanoparticles isostatically pressed into it may also be sintered in anoven. The sintering may be carried out in a reduced pressureenvironment, vacuum, a reducing gas environment (5% H₂ in argon), orother suitable gas environment for the sintering process. The sinteringtemperature, heating and cooling rates, and times for the sinteringprocess will depend upon the materials sintered and may be changed toaffect the final product pore size, strength, and surface area of theformed sintered porous composite material as would be obvious to thoseskilled in the art. The final sintered porous composite material may betreated one or more times with nanoparticles after sintering to buildmultiple layers of materials or graded porosities.

[0053] As shown in FIG. 3, a sintered porous composition including aporous sintered nanoparticle material 312 within pores of the base 320and a porous sintered nanoparticle layer from 340 to 350 atop the basesurface 350. The porous sintered composite material thickness extendsfrom a side 370 of the porous base material 320 to a height 340 above asurface 350 of the porous base material 320. The porous sinterednanoparticle layer of material between 350 and 340 atop the porous basematerial 320 includes the nanoparticle material 330. The porous sinterednanoparticle material 312 penetrates a portion of the pores 300 of thebase 320. The porous sintered nanoparticle material within the basepores 312 and the porous sintered nanoparticle material from 340 to thebase surface 350 forms a substantially continuous structure. The poroussintered nanoparticle material layer includes interconnected pores 310that are in fluid communication with and interconnected to the pores ofthe porous base 300. The porous sintered nanoparticle material in thepores of the base 312 may sintered to the base material 380, thenanoparticles may only sinter to each other whereby the sinterednanoparticle material mechanically interlocks with the interconnectedpores in the porous base structure 382, or a combination of these. Thepowdered nanoparticles on the one or more surfaces of the porous basemay sinter to the base material at its top surface 350, sinterednanoparticles, or sinter to both. Preferably the porous compositematerial comprises nanometer and sub-nanometer sized pores 310throughout the sintered nanoparticle material. The porous compositematerial may comprise layers of different nanoparticle materials suchassize, shape, composition and combinations of these.

[0054] The porous composite materials of the present invention formasymmetric structures. FIG. 2. is an illustration of a cross section ofa porous composite material with an asymmetric structure formed byisostatically compressing powdered nanoparticles 230. This materialincludes a porous base 220, nanoparticles 230 within and penetrating atleast a portion of the interconnected base pores 200 from the topsurface 250 of the base to a depth 260, and a fine layer ofnanoparticles with interconnected pores 210 from the top surface of thebase 250 to a thickness or top surface of nanopowder material layer 240above the base surface. The thickness of the porous composite materialextends from a side 270 to the top surface of the nanopowder materiallayer 240. The pores 210 of the powdered nanoparticles are in fluidcommunication with the base pores 200. This porous composite materialmay be made by isostatically pressing a powdered nanoparticle 230, forexample but not limited to nickel, into a frit or a green form of aporous base material. Alternatively, the porous composite material maybe made by flowing a source of powdered nanoparticle 230 in a fluid intoa frit or a green form of a porous base material an capturing thepowdered nanoparticles within the base pores 200 and on its surface 250.The porous composite material in FIG. 2 may be sintered to form thesintered porous composite material shown in FIG. 3.

[0055] Filter elements from the porous sintered composite material maybe formed into a variety of shapes to control surface area, pressuredrop, and mechanical strength. Shapes may include but are not limited todisks or tubes, pleated structures, or electrodes comprising thesintered porous composite materials of the present invention. Thesestructures may be welded, compression fit, epoxied, fusion bonded to athermoplastic, or otherwise fixtured or secured into a housing. Thehousing may be in the form of a tube, a canister, or other shapesuitable for its intended use. The housing may comprise a void volumeand a variety of inlet and outlet ports for fluid flow. The ports may bepositioned about the housing as required for its use and the ports mayinclude but are not limited to metal seals, compression fittings, barbs,or welded fittings. The fluid may then be made to pass through thehousing comprising the sintered porous composite material element forfiltration, purification, catalysis, sensing, or combinations of these.

[0056] In one embodiment of the present invention, one or more sinteredporous composite material elements or other porous filter elements maybe welded or press fit into a housing which further comprises a bedmaterial as shown schematically in FIG. 6; preferably the poroussintered nanoparticle material layer of the sintered porous compositefilter element has pores which are nanometer sized. The housing and theelement may be covered with a bed of material that further effectsparticle and or contaminant removal from fluids which flow through thebed material.

[0057] The sintered porous composite materials of the present inventionmay be bonded or joined to one or more housing members which provide anintegral seal with the housing members while retaining the porestructure and size of the sintered porous nanoparticle material withinthe base and the sintered porous nanoparticle layer on the surfaces ofthe base in the sintered porous composite material. The sintered porouscomposite material filter element and the one or more housing membersmay be joined by press fitting, compression fitting, a metal seal,welding, or by use of a graded seal using a polymer or glass. Heating ofa sintered porous composite material filter element to form such a seal,for example a glass to metal seal or a weld, with one or more housingelements may result in localized heating of the sintered porouscomposite material element and fusion or melting of the entrained orsintered nanoparticles within the porous composite material. The area ofthe sintered porous composite material between the porous compositematerial and one or more housing members with which it is to be bonded,may if necessary be further be sealed, impregnated, or filled to reduceparticle penetration through pores in these areas. Preferably thefilling, impregnation, or closure of these pores provides asubstantially uniform particle retention across the sintered porouscomposite material joined to the housing as determined by a mean poreflow test and shown by a sharp transition between diffusive flow andbulk flow in a bubble point test. The pores near the interface of a weldor glass seal area may be impregnated, closed, or filled by a variety oftechniques as would be known to those skilled in the art including butnot limited to mechanically sealing the membrane in the area near wherethe porous composite material is heated, use of a gasket or o-ring atthe area near where the porous composite material is heated; use of highheat capacity gas such as helium to cool the heated site and preventfusion of the entrained nanomaterials, use of adhesives or polymers tophysically seal the heated treated areas, or impregnation ofnanoparticles into the porous composite material in the heat treatedarea.

[0058] Examples of suitable bed materials include but are not limited topowders, fibers, fiber mesh, aerogels, foams, woven matrices, flat sheetmembranes, depth filtration media, and combinations of these. Suitablebed materials include but are not limited to chemically compatiblemetals, metal alloys, chemically reactive or chemically functionalizedparticles, metal oxides or hydroxides, ceramics, polymers, salts, carboncomprising materials, semiconductors, and combinations of these. Bedmaterial examples include Ni powder, like INCO type 255, 316 L stainlesssteel powder, alumina powder, silicon nitride powder, quartz fibers, andpolytetrafluoroethylene powder. The particle size of the bed materialshould suitable to provide a void free packing and sufficient particleor contaminant removal in the interstices in the bed. Particle sizes forthe bed material may range from 3 millimeters to 0.2 microns. For someapplications, such as supercritical fluids, bed particles may have asize in the range of 0.2 μm to 30 μm diameter, fibers may also havediameters in the range of 0.2 μm to 30 μm and lengths of from 0.2microns to 3 millimeters. The distribution of particle size or shapewill depend upon the characteristics of the bed; for graded beds a largeparticle size, material composition, and or shape may be use. For otherbeds a particle distribution may be for example but not limited to 5%rms of the particle diameter.

[0059] Bed materials may be chosen for their ability to removecontaminants from the fluid. Examples of molecular contaminants caninclude water, metals, and organics. For example, supercritical carbondioxide can become contaminated with hydrocarbons from pumps and theapparatus. In the semiconductor industry it is highly desirable toremove any hydrocarbon from fluids used to clean or react with wafers.Materials useful for removing these contaminants may be adsorptivematerials like zeolites, alumina, carbon and activated carbon beds forremoval of hydrocarbons. Other materials include those disclosed in U.S.Pat. No. 6,361,696 the contents of which are incorporated herein byreference in their entirety. The removal of contaminants from the fluidmay be determined off-line using techniques known to those skilled inthe art. For example gas chromatography utilizing a flame ionization orelectron capture detector may be used for measuring hydrocarbon andcarbon monoxide concentration in fluids below 1 part per million, totalresidues in the fluid may be measured to nanogram levels utilizing aquartz microbalance or surface acoustic wave device on a suitablyconcentrated fluid sample; moisture may be determined utilizingcommercially available electrolytic moisture analyzers, metals may bedetermined utilizing ICP-MS on concentrated samples with nitric acid.The methods and materials disclosed in SEMI C3.57-0600 may also be usedin the analysis of carbon dioxide gas and residues from theconcentration of contaminants or purified fluids.

[0060] Preferably the amount of hydrocarbon in a fluid treated with anapparatus of the present invention, for example supercritical carbondioxide, is less than about 100 parts per billion (mole/mole) and theamount of moisture is less than about 100 parts per billion (mole/mole)based on the analysis of a gaseous sample of the fluid. Embodiments ofthe present invention may be used to remove particles from supercriticalfluids such as but not limited to carbon dioxide. As shown in FIG. 7,the porous composite material and the porous composite material with apacked bed of material such as described in Examples 2 and 6 may be usedto reduce the number of particles on a substrate cleaned withsupercritical CO₂. Preferably the number of particles greater than about0.2 micron in size remaining on a substrate cleaned with a about 5liters of supercritical carbon dioxide fluid filtered with a sinteredporous composite material or a device including a sintered porouscomposite material element of the present invention and a packed bed ofmaterial is less than about 300 counts on a 200 mm Si wafer, and morepreferably less about 100 counts on a 200 mm Si wafer as measured bylight scattering measurements of the treated substrate.

[0061] The void volume of the housing is filled with bed material andpacked to a density sufficient to capture particles and contaminantsfrom a fluid to be treated and to also prevent voids, bypass, andprevent restricted fluid flow or pressure drop. The bed maybe packed forexample by pressing, vibrating, or tamping the bed material in thehousing having a first filter element in place. Packing densities mayrange from 1 to 90%. Graded or mixed beds comprising different bedmaterials, different material morphology, different size, andcombinations of these may be used. All or a portion of the void volumemay be filled with the bed material and a second filter element bondedor press fit into the housing to secure the bed material. The deviceincluding a sintered porous composite material filter element with alayer of porous sintered nanoparticle material having nanometer sizedpores and a bed of material for contaminant removal has an LRV of atleast 2 for 0.2 μm particles, preferably an LRV of at least 4 for 0.2 μmparticles, more preferably an LRV of at least 2 for 0.05 μm particles,and most preferably an LRV of at least 4 for 0.05 μm particles in waterand a pressure drop in water of less than 500 psi/slpm; preferably lessthan about 50 psi/slpm; and most preferably less than about 5 psi/slpmfor a 15 cm² sintered porous composite filter element. One skilled inthe art could determine the bed material and packing density required toeffect a required pressure drop at a given fluid flow rate using flowmeters and pressure gauges; particle removal from fluids used to cleansubstrates could be determined by laser surface scanners.

[0062] As illustrated in FIG. 6, one embodiment of the present inventionincludes a housing 600, a second filter element or frit 620, a bedmaterial 640, and an first filter or frit 660. The second filter element620 may be made for example a using a porous metal filter element withthe pore size of the element less than about 20 microns as is describedin U.S. Pat. No. 5,487,771 the teachings of which are incorporated byreference herein in their entirety. Preferably the second filter element620 is a porous composite material, more preferably 620 is a sinteredporous composite material having a sintered porous nanoparticle materiallayer with nanometer and or sub-nanometer sized pores. The second filterelement 620 may be welded 680 between two metal parts, preferablystainless steel parts, and more preferably materials such as but notlimited to 316L or Hastalloy. One of the metal parts may be a tube usedfor a housing 600 and has an end for welding or bonding to the secondfilter element 620 while the second part may be a fluid connection 670.The fluid connection 670 may consist of but is not limited to variouspipe fittings, a tube stub for welding, a compression fitting, or asshown in FIG. 6 a fluid fitting 670 such as a ¼″ “VCR” male typefitting. The fluid connection 670 also has an end for welding or bondingto a the second filter element 620. The length, diameter and shape ofthe housing 600 define the volume of the bed as shown in FIG. 6. Thehousing 600 for the bed 640 may be any acceptable shape or volume. Thesecond filter element 620 fits between the two metal parts and the threeare welded or bonded 680 into one solid subassembly. The subassembly maythen be filled with a fine powder or bed material 640 that is preferablya nickel powder with particles ranging in size from about 0.2 to 30microns in diameter vide supra. The powder is tapped and or vibrated andpacked into the bonded subassembly until the desired weight and orpacking density of the bed powder 640 is achieved. The desirability of apacked bed of powder 640 may be determined by its pressure drop and orcontaminant retention; lower pressure drop being achieved for example bylower packing density of the bed, shorter bed length, and largerdiameter beds; higher contaminant retention being achieved with higherpacking densities and longer beds. Once the bed of powdered material hasbeen formed in the sub-assembly, then, a first open porous metalstructure or filter 660 with a suitable pore size, of for example ofabout 20 microns or greater, is pressed, welded or bonded 674 into thesubassembly to hold or retain the bed material 640 in place. Finally, afluid fitting 672 having one end for bonding to the first filter 660 andhousing 600 and a second end for connection to a fluid fitting asdescribed previously, is welded 674 to the subassembly as illustrated inFIG. 6. In one embodiment, the second filter element 620 may be made byfirst welding into the housing 600 a base filter element which may besubsequently treated by impregnation, entrainment, or penetration with apowdered material, preferably a powdered material comprising nanometersize particles, to form the second filter element as a porous compositematerial having nanometer sized pores. Optionally, the base filter 620element may be covered with a nanoporous membrane (a membrane havingnanometer size pores) to form the second filter element having nanometersized pores.

[0063] Embodiments of the present invention can be used for filteringand or purification of a wide variety of fluids including supercriticalfluids and liquids. In some applications it may be desirable to separatethe sintered porous composite filter element from the purification bedand place them into separate housings fluidly connected to one anotherby a conduit. Such an arrangement makes replacement of one component orregeneration of it easier and less costly. The materials of constructionof the device may be chosen to make them useful for filtration orpurification over a wide range of temperatures under which the sinteredporous composite materials and or bed materials are thermally andmechanically stable. For example, liquid helium, liquid nitrogen, liquidcarbon dioxide, as well as heated liquids may be filtered or purifiedwith embodiments of the present invention. Preferably the temperature ofthe liquid or fluid does not alter the mechanical properties or poresize of the sintered porous composite material filter or bed material.Preferably the temperature is below about 300° C. The wide range ofthermal stability of the elements of the present invention also permitsfluids having a wide range of viscosities to be treated. The viscosityof the fluid may be that which gives an acceptable fluid flow andpressure drop for the filter and bed material in the application. Insome cases the viscosity of a liquid may be reduced by heating followedby filtration or purification. A method for removing contaminants from afluid includes providing device with a sintered porous composite elementand a bed of material in a housing as shown in FIG. 6, and flowing afluid with contaminants, such as but not limited to hydrocarbons,moisture, particles, or a combination of these through the device toremove one or more contaminants and particles from the fluid.

[0064] In part, the following non-limiting examples and data illustratevarious embodiment and features relating to the compositions, methods,and components of the present invention. While various aspects of thepractice and use of this invention are illustrated by these examples andby the components and processes used, it will be understood by thoseskilled in the art that substantially comparable results may be obtainedwith various other reagents, apparatus and processes which arecommensurate and within the scope of the embodiments of this invention.

EXAMPLE 1

[0065] This example illustrates the formation of a composite porousmaterials using an aerosol to entrain nanoparticles into the porous basematerials.

[0066] Number 1: disc: The porous base substrate material was a sintered255 nickel disc, diameter: 1.5″, thickness 0.1″, porosity 51.5%, bubblepoint in water was 15 psi, and which had an 11.5 psi differentialpressure loss at 15 slpm air flow. The membrane had a starting mass of18.103 grams and was fixtured for aerosol treatment. Treat fixturedmembrane with 60 nm nickel nanoparticle aerosol (approx. challenge:2E+07 particles/min) at 15 slpm air flow rate for 14.5 hours to form aporous composite material. The resulting product was sintered at 600° C.for 45 minutes in 5% H₂/Argon. The mass of the sintered porous compositeproduct was 18.116 grams and had an 13 psi differential pressure loss at15 slpm air flow. The bubble point of the formed sintered porouscomposite material in water was about 15 psi.

[0067] Number 2: tube: The porous base substrate material was sintered255 Ni tube, 1.38″ long, OD 0.635″, wall thickness 0.065″, porosity 64%,bubble point in water 10 psi, and which had a 7.5 psi differentialpressure loss at 30 slpm in air flow. This tube was welded into“sub-assembly” and had a starting mass of 38.6965 grams. Thissub-assembly was treated with 60 nanometer diameter nickel aerosol(approx. challenge: 2E+07 particles/min) for 48 hours at 20 slpmnitrogen flow rate. The resulting porous composite material product wassintered at 575° C. for 40 min 5% H₂/Argon to form the sintered porouscomposite material. The mass of the sintered porous composite materialproduct was 38.722 grams and had an 8.0 psi differential pressure lossat 30 slpm nitrogen flow. The bubble point of the formed sintered porouscomposite material in water was 11.5 psi.

[0068] Number 3: tube: The porous base substrate material was a sintered255 Ni tube, 1.38″ long, OD 0.635″, wall thickness 0.065″, porosity 54%,bubble point in water was 15 psi, and which had a 12 psi differentialpressure loss at 30 slpm in air. This tube was welded into“sub-assembly”, and had a starting mass of 39.4557 grams. Thesub-assembly was treated with an aerosol as above but with 210H Nipowder for 7 hours at 20 slpm nitrogen gas flow. The resulting porouscomposite material product was sintered at 560° C. for 35 min 5%H₂/Argon. The mass of the sintered porous composite material product was39.469 grams; and had a 13 psi differential pressure loss at 30 slpmflow of nitrogen. The bubble point for the sintered porous compositematerial in water 15.5 was psi.

EXAMPLE 2

[0069] This example shows how a porous composite material may be madeusing the isostatic method which is then sintered to form a sinteredporous composite material. An example of such a sintered porouscomposite material is illustrated schematically in FIG. 3.

[0070] A mold with ID 0.850″ and 7″ long and a steel mandrel 0.550″diameter was filled with 45 grams of 255 Nickel powder, Fisher size (2.8microns). This was isostatically pressed at 500-1000 psi. The dimensionsof this green form were: OD: 0.708″, ID 0.550″, length 7″. The greenform and mandrel was carefully placed into a new mold with ID: 0.800″.This mold was filled with 9.5 grams of 210H nickel powder (Fisher size0.3 microns) and isostatically pressed at 500-1000 psi. This layeredgreen form (dimensions: OD: 0.745″, ID 0.550″ length 7″, weight: 54.5grams), was sintered in vacuum and a reducing atmosphere 5% H₂ in Argonat 575° C. for 30 minutes. The sintered porous composite tube had afinal OD: 0.685″ and total wall thickness: 0.082″ (fine layer approx.0.005-0.015 inches or 127-381 micrometers). The tube was cut intoindividual tubes: Length: 1.38″, Weight: 11 grams, density was 3.13grams/cc. Gas flow testing of the dry cut sintered porous compositetubes showed that they had a 21 psi differential pressure drop at a gasflow rate of 30 slpm of air. FIG. 4 shows the pore symmetry test resultsof this sintered porous composite tube; its bubble point wasapproximately 50 psi in H₂O. FIG. 5 shows that the particle retention ofthe sintered porous composite tube is at least 2 LRV for a 0.2 micronsized particle. The fine layer as illustrated for example between 340and 350 in FIG. 3, typically has a density of between 3.0-4.5 g/cc andis preferably around 3.8-4.2 g/cc (52-57% porous). The porous basesubstrate, for example 120 in FIG. 3, is typically around 64% porous,but may range from about 60 to about 70%.

[0071] Liquid particle retention, as shown in FIG. 5, was measured bydetermining the number of particles captured by sieving, i.e. theparticles are captured by the sintered porous composite membrane only ifthey are larger than the porous composite membrane's pore. The liquidretention test was performed in DI water using a challenge of PSL beadsof known size. The PSL bead mixture is diluted 1:100 by volume. Triton®X surfactant was added, for example 20% by volume, to remove surfacecharges from the PSL beads and permit sieving retention of the membraneto be determined. For example, 40 microliters of 0.137 micron PSL beadsis added to 4,000 microliters of water containing 20% Triton® X toprepare a particle test solution. The water test flow rate through thesintered porous composite membrane was set to 140 ml/min and opticalparticle counters capable of measuring 0.03 to 0.2 micron size particleswere used to measure particle concentration and size. The particleconcentration of the PSL bead/surfactant solution is measured before thesintered porous composite membrane filter was placed on the test stand.Prior to challenging the filter, the background counts were recorded.The pressure drop as a function of water flow rate through this sinteredporous composite membrane in water is shown in FIG. 10; its pressuredrop as a function of flow in supercritical CO₂ is shown in FIG. 11.

EXAMPLE 3

[0072] This example illustrates the formation of a porous compositematerial using a slurry of nanoparticles in a liquid to entrainnanoparticles into a porous base material. The substrate material was a255 Nickel sintered tube, 1.38″ long, OD 0.635″, wall thickness 0.065″,porosity 54% porous that was welded into a “sub-assembly.” The weight ofthe tube and subassembly was 39.6728 grams. The tube had a bubble pointin water of 15 psi, and a 12 psi differential pressure loss at 30 slpmflow of air.

[0073] A mixture of 8 g off INCO Ni powder type 110 (ref 1.0 micronFischer particle size) in 800 ml of IPA (for low surface tension) wasprepared. This mixture was placed in a pressure vessel and the mixtureforced to flow through the welded “subassembly” at 30 psig to “filter”600 ml of the dispersion. The coated subassembly was “dried” by flowingair at 15 psi for 5 minutes through the coated subassembly, this porouscomposite material subassembly was then dried in air using an oven at100° C. for ½ hour.

[0074] The dried subassembly was sintered at 525° C. for 1 hour. First 5minutes of sintering was performed in vacuum, next 20 minutes 95%argon/5% hydrogen, and then balance of time (35 minutes) in a vacuumatmosphere. The sintered porous composite material subassembly wasremoved from the oven and test/measured. The mass of the added layer ofnickel INCON powder was 2.144 g with a porous sintered nanoparticlelayer thickness of approx. 300 micron and a porosity of approx. 54%. Thebubble point of this sintered porous composite in water was 22 psi andit had a 17 psi differential pressure loss at 30 slpm flow of air.

EXAMPLE 4

[0075] In this prophetic example, the sintered porous materialcomposition is going to be used as a membrane to filter a super criticalfluid. Supercritical fluids are being used to replace a variety oforganic and inorganic solvents used in industrial cleaning, purificationand re-crystallization operations. The density of supercritical fluidsis usually between 0.25 and 1.2 g/ml and is strongly pressure andtemperature dependent. The solvent strength increases with density;changing the pressure or temperature enables the solvating properties ofthe supercritical fluid to be changed. Supercritical fluids can act as acarrier for co-solvents, like methanol, which can be added tosupercritical fluids to tailor the solubility of various solids into thesupercritical fluid carrier phase. Diffusion coefficients of solutes insupercritical fluids are ten-fold greater than in the correspondingliquid solvents, and are about three orders of magnitude less than thecorresponding diffusion coefficients in the gases. The high diffusivityof solutes in supercritical fluids decreases the resistance to masstransfer of solutes into the supercritical fluid as compared to liquids.The surface tension of supercritical fluids is essentially like that ofthe gas and so supercritical fluid can flow into and through narrow poreor geometries with little pressure loss compared to liquids.

[0076] Water and carbon dioxide are common supercritical fluids used forextraction and their solvation properties. Supercritical water is astrong oxidant, especially when oxygen is dissolved into it, and isuseful for oxidizing and eliminating toxins and organic compounds fromwaste media and substrates. Carbon dioxide, which is a super criticalfluid above 31.2° C. and 1071.3 psi, is being used in cleaning andstripping operations for advanced integrated circuit manufacturingprocesses as well as food and beverage extraction processes. Because ofits low surface tension and viscosity, supercritical CO₂ can easily flowinto and clean trenches and vias in microelectronic devices. In additionsupercritical fluids may be used in the manufacture of nanoscalebiological and pharmaceutical materials such as proteins, DNA, variouscells, and drugs in the form of an aerosol. In these applications, afilter capable of removing undesirable foreign matter, such as harmfulspores and harmful bacteria, is highly desirable.

[0077] Systems utilizing supercritical fluids may operate in a closedloop. The supercritical fluid contacts the substrate or sample to beextracted or cleaned in a chamber, the substrate or sample may beremoved from the chamber after cleaning, and the supercritical fluidcontaining the extracted material or particles is returned to acollection vessel. A sintered porous composite material with a pore sizeof about 10-200 nanometers made by the methods disclosed herein may bewelded to a housing to form a filter element which may then be connectedin fluid communication with the closed loop supercritical fluid system.The sintered porous composite material filter element in the housing maybe used to remove contaminants like particles and dissolved materialsfrom the fluid. The removal may comprise removing the one or morecontaminants from the fluid by the sintered porous composite material bysieving filtration. Other such contaminant removal acts may comprisefiltration, purification with a bed of a purifier material, andcombinations of these acts to remove dissolved contaminants and orfilter particles from the supercritical fluid before it is returned tothe extraction chamber for further use. Other acts, such as changing thetemperature and or pressure of the system may be used to affect thesolubility of contaminants (e.g. cause precipitation) in the fluid andaid in the separation of contaminants from the supercritical fluid.After treatment, makeup gas or co-solvent may be added to supercriticalfluid in the system. The use of the sintered porous composite materialas a filter may be used to extend the useful life of the extractionfluid and may lead to cleaner substrates with reduced particle counts.

EXAMPLE 5

[0078] In this example a depth filter or purifier is made. The purifierconsists of a sintered porous composite filter element such as the onedescribed in Example 2 which is welded into a housing, the housing sizedto be equal to or greater in length and diameter than the filterelement. The void volume of the housing is filled with a Ni powder, likeINCO type 255. Enough powder is placed into the housing to prevent voidsand bypass, but not so much as to greatly restrict fluid flow.

[0079]FIG. 6 is an illustration of the depth filter of the exampleillustrating a housing, inlet filter 660, outlet sintered porouscomposite filter element 620, a packed bed of powdered material 640between the filter elements, and fittings for connection to fluid flowcircuit.

EXAMPLE 6

[0080] In this example a sintered porous composite metal filter elementsuch as described Example 1 is welded between two stainless steel parts.One part (called the “outlet”) consists of a short ¾″ tube with a ¼″“VCR” male type fitting, the other is a ¾″ tube 1″ long which is thehousing. The filter element fits between the two part and they arewelded into one solid “subassembly.”

[0081] The subassembly is then filled with a fine Ni powder. In thiscase, INCO type 255 with a particle size of 1-3 micron. The Ni powder istapped and vibrated and packed into the subassembly until the desiredweight of bed material is achieved. Then, an open porous metal structurewith a pore size of 20 microns or greater is pressed into thesubassembly to hold the powder in place. Finally, an inlet fittingsimilar to the already described outlet fitting is welded to thesubassembly, resulting in a totally enclosed filter/purifier having abed of nickel powder.

[0082] When 8 grams of Ni 255 are placed in the subassembly housing andsealed with an inlet fitting, the resulting bed has a density of 1.6g/cc and a void volume of 84%. The pressure drop measured was 18 psi(1.2Bar) at a flow of 2 slpm air with the outlet pressure at atmosphere.

[0083] When 16 grams of the Ni 255 are placed in the subassembly housingand sealed with an inlet fitting, the resulting bed has a density of 3.2g/cc and a void volume of 64%. The pressure drop measured was 64 psi(4.3Bar) at a flow of 2 slpm air with the outlet pressure at atmosphere.

[0084] When 10 grams of Ni 255 powder was tapped into the subassemblyhousing and sealed with an inlet fitting, the resulting bed had adensity of 1.30 g/cc and a void volume of 85%. The pressure dropmeasured in air was 52 psi at a flow of 2 slpm with outlet toatmosphere. This pressure drop is lower than the example above since theamount of bed material is less, resulting in a lower packing density.

[0085] A plot of pressure drop versus mass flow rate of supercriticalcarbon dioxide is shown in FIG. 14.

EXAMPLE 7

[0086] In this example, the sintered porous composite material filterelements of Example 2 and Example 6 were installed on a SupercriticalCO₂ wafer cleaning tool and the particle concentration remaining on testwafers cleaned by the tool were measured. The particle data is forparticles >0.2 micron using a KLA-Tencor surfscan. The results are for200 mm diameter substrates and cleaning done using about 5 liters ofsupercritical CO₂ per run. In FIG. 7, the sintered porous compositematerial filter of Example 2 show the wafer counts on the wafer withjust the porous filter element and the sintered porous compositematerial filter with a bed of material of Example 6 shows the particlecounts when the present invention is used to filter the SC CO₂ fluid.The graph shows that sintered porous composite material filter ofExample 2 results in fewer particle counts on the wafer and that thedevice comprising a packed bed of material of Example 6 also results inthe reduction of particle counts on the wafer. Based on the graph thedevice of the present invention is capable of filtering supercriticalcarbon dioxide for cleaning a wafer that leaves less than about 300particles with a size greater than 0.2 micrometers on the wafer when 5liters of supercritical carbon dioxide are used.

EXAMPLE 8

[0087] In this example, a tube sintered porous composite filter elementmade as in Example 2 is welded into a housing and the interface areanear the sintered porous composite material and the weld is sealed. Thepore symmetry test for this filter element, FIG. 8, shows a sharptransition between the diffusive flow and bulk flow of gas. The particleretention of the welded and sealed filter element is about 4 LRV for 0.2μm polystyrene latex beads in water as shown in FIG. 9.

EXAMPLE 9

[0088] This example illustrates a sintered porous composite material ofthe present capable of removing 0.05 μm polystyrene latex beads in wateras shown in FIG. 13, the porous sintered composite material made by theisostatic method.

[0089] A mold with ID 0.850″ and 6″ long and a steel mandrel 0.655″diameter was filled with 28 grams of 255 Nickel powder (Fisher size 2.8microns). This was isostatically pressed at 5000-6000 psi. The dimensionof this green form were: OD: 0.722″, ID 0.655″, length 6″. The greenform and mandrel was carefully placed into a new mold with ID: 0.800″.This mold was filled with 7 grams of 210H nickel powder (Fisher size 0.3microns) and isostatically pressed at 7000-8000 psi. This layered greenform (dimensions: OD: 0.735″, ID 0.655″ length 6″, weight: 35 grams),was sintered in vacuum and a reducing atmosphere 5% H₂ in Argon at450-500° C. for 30 minutes. The sintered porous composite tube had afinal OD: 0.685″ and total wall thickness: 0.036″ (fine layer approx.0.003-0.006″). The tube was cut into individual tubes: Length: 1.38″,Weight: 7.5 grams, density was 4.5 grams/cc. Gas flow testing of the drycut tubes with a flow area of 16 cm² showed that they had a 27 psidifferential pressure drop at a gas flow rate of 20 slpm of air.

[0090] The fine layer had a porosity of around 37% and the substratearound 51% (may range from about 45 to about 55%). Bubble point testingof this material was conducted in 60/40 IPA solution as shown in FIG.12, Particle retention was conducted in DI water using PSL beadsneutralized so the filtration mechanism was purely sieve type, theresults of the particle retention testing are illustrated in FIG. 13 andshow that the material has an LRV of at least 4 for 0.05 μm particlesand an LRV of at least 5 for 0.2 μm particles.

[0091] Although the present invention has been described in considerabledetail with reference to certain preferred embodiments thereof, otherversions are possible. Therefore the spirit and scope of the appendedclaims should not be limited to the description and the preferredversions contain within this specification.

What is claimed:
 1. A sintered porous composite material comprising: aporous base material; and a layer of porous sintered nanoparticlematerial, said layer of porous sintered nanoparticle material on one ormore surfaces of the porous base and penetrating a portion of saidporous base material, said porous sintered nanoparticle material havingpores smaller than the pores in said porous base material.
 2. Thesintered porous composite material of claim 1, wherein said sinterednanoparticle material is comprised of metals, metal alloys, and mixturesof these materials.
 3. The sintered porous composite material of claim 1wherein said porous sintered nanoparticle material includes nickel. 4.The sintered porous composition of claim 1 wherein said porous sinterednanoparticle material includes sintered dendritic nanoparticles.
 5. Thesintered porous composite material of claim 1 further comprising a gas,liquid, supercritical fluid or mixtures of these in the pores of saidporous sintered nanoparticle material.
 6. The sintered porous compositematerial of claim 1 further comprising: a housing wherein said sinteredporous composite material is bonded to said housing, and wherein saidhousing with the bonded sintered porous composite material ischaracterized in that is has a sieving LRV of at least 2 for 0.2 μmparticles in a fluid.
 7. A filter element comprising: a porous basematerial and a layer of porous sintered nanoparticle material formed bysintering a powdered nanoparticle material layer penetrating a portionof said porous base, said layer of porous sintered nanoparticle materialon one or more surfaces of the porous base, said porous sinterednanoparticle material having pores smaller than the pores in said porousbase.
 8. The filter element of claim 7, wherein said sinterednanoparticle material is comprised of metals, metal alloys, and mixturesof these materials.
 9. The filter element of claim 7 further comprising:a housing wherein said filter element is bonded to said housing, andwherein said housing with the bonded filter element is characterized inthat is has a sieving LRV of at least 2 for 0.2 μm particles in a fluid.10. A sintered porous composite material comprising: a porous sinteredmetal base material; a layer of porous sintered nanoparticle material onone or more surfaces of a porous base and penetrating a portion thereof;and, a porous sintered nanoparticle material within the base pores,forming a substantially continuous structure and having interconnectedpores smaller than the pores in said porous base material.
 11. Thesintered porous composite material of claim 10, wherein said sinterednanoparticle material is comprised of metals, metal alloys, and mixturesof these materials.
 12. A method of making a porous composite materialcomprising: sintering a layer of powdered nanoparticles on a porous basematerial to form a layer of porous sintered nanoparticle material onsaid base, said layer said of powdered nanoparticle on one or moresurfaces of the porous base and penetrating a portion of said porousbase material.
 13. The method of claim 12 further comprising the act offorming said layer of powdered nanoparticles on said porous basematerial by isostatically pressing said powdered nanoparticles into saidporous base.
 14. The method of claim 12, wherein said sinterednanoparticle material layer is comprised of metals, metal alloys, andmixtures of these materials.
 15. A method for removing material from afluid comprising: flowing a fluid having said material therein throughthe sintered porous composite material of claim 1 wherein the saidsintered porous composite material removes said material from the fluid.16. The method of claim 15 wherein said material is removed by particlecapture.
 17. The method of claim 15 wherein said fluid is asupercritical fluid.
 18. A supercritical fluid that deposits less than300 particles greater than 0.2 microns in size on a 200 millimeterdiameter substrate when 5 liters of said supercritical fluid arefiltered through the porous composite material of claim
 1. 19. Anapparatus for removing contaminants from a fluid stream comprising: ahousing for containing a bed material; a second filter element that is asintered porous composite material having nanometer sized pores, saidsecond filter element secured to said housing to permit fluid flowthrough the apparatus, the bed material, and said second filter element,said second filter removing particles from said fluid stream; a bed ofmaterial covering said second filter element and contained within saidhousing, said bed removing contaminants from said fluid stream; and afirst filter element secured to the housing that retains the bedmaterial within the housing between the first filter element and thesecond filter element, said first filter element permitting fluid flowthrough the apparatus.
 20. A supercritical fluid with less than 50particles per milliliter, said particles having a size of 0.2micrometers or less.
 21. A sintered porous composite materialcomprising: a porous base material; and a layer of porous sinterednanoparticle material, said layer of porous sintered nanoparticlematerial on one or more surfaces of the porous base and penetrating aportion of said porous base material, said porous sintered nanoparticlematerial having pores smaller than the pores in said porous basematerial; said porous composite material is characterized in that is hasan LRV of at least 2 for a 0.2 μm or larger particles in water.
 22. Thesintered porous composite material of claim 21 wherein said material ischaracterized in that it has an LRV of at least 4 for a 0.2 μm particlechallenge in water.
 23. The sintered porous composite material of claim21 wherein said material is characterized in that it has an LRV of atleast 2 for a 0.05 μm particle challenge in water.
 24. The sinteredporous composite material of claim 21 wherein said material ischaracterized in that it has an LRV of at least 4 for a 0.05 μm particlechallenge in water.
 25. The sintered porous composite material of claim21 having a pressure coefficient in nitrogen of less than
 250. 26. Thesintered composite material of claim 21 able to support a differentialpressure across the material of greater than 60 psi.
 27. The sinteredcomposite material of claim 21 wherein the thickness of the poroussintered nanoparticle material is less than 100 microns.
 28. Thesintered composite material of claim 21 wherein the porous sinterednanoparticle material include particles less than 1000 nm.